In the muck beneath the ocean floor, there’s something alive. Lots of somethings. But don’t worry: You’ll never see them. Instead, these tiny, one-celled germs are content to hunker down in very old clay, for a very long time, eating just enough to stay alive.

“These organisms are so different from anything we know,” says Hans Roy, a biologist from Aarhus University in Denmark. He has been studying microbes that live beneath the Pacific Ocean, near the equator. Recently, he and other scientists published a study in the journal Science that contained a surprising observation: These organisms may live for an astonishingly long time — perhaps millions of years.